It is known to use, as the transfer pump for use in a fuel system, a vane pump which is arranged to operate at a speed associated with engine speed. The vane pump may be provided with a regulator of the type illustrated in FIG. 1, comprising a sleeve 10 within which a piston member 12 is slidable. The piston member 12 is biased by means of a spring 14 and is moveable under the action of the fuel pressure at the outlet of the transfer pump. When the piston member 12 is in the position shown in FIG. 1, the piston member 12 closes an outlet 16 defined within the sleeve 10 to prevent fuel flowing through the outlet 16 to the inlet side of the transfer pump. The piston member 12 is moveable within the sleeve 10 to a position in which the outlet 16 is open to allow fuel to return to the inlet side of the transfer pump.
The degree to which the outlet 16 is opened depends upon the magnitude of the pressure applied to the piston member 12. By appropriate selection of the size of the piston member 12 and the spring 14, the regulator can be set in such a manner that the outlet pressure of the transfer pump, as regulated by the pressure regulator, is related to the engine and pump speed in a desired manner.
The pressure at the outlet of the transfer pump, as modified by the pressure regulator, is used to control various aspects of fuel system operation. For example, in some arrangements the pressure at the outlet of the transfer pump, as modified by the pressure regulator, is used to adjust the maximum fuelling level of a high pressure fuel pump with which the transfer pump is associated. However, it has been found that setting of such transfer pumps is difficult as the relationship between the outlet pressure and speed varies from pump to pump as a result of production variations.
It is an object of the present invention to remove or alleviate this problem.
By way of background to the present invention, CH 690 191 A5 describes a pressure relief valve for relieving fluid pressure within a load chamber. The pressure relief valve is of the type having a valve needle biased towards a valve seating by means of a spring load. The valve needle is caused to lift away from the valve seating in the event that fluid pressure within the load chamber exceeds a predetermined amount, thereby opening communication between the load chamber and a low pressure drain. The pressure relief valve also includes a piston which is exposed to fluid pressure within a control chamber and which is engageable with a movable piece to alter the spring load acting on the valve needle. By varying fluid pressure within the control chamber, the fluid pressure at which the valve needle is caused to lift from its seating can be varied. The pressure relief valve is of the “plug-in-hole” type in which the valve needle extends into an opening in an insert which defines the valve seating. If fluid pressure within the load chamber is less than the predetermined relief pressure, the pressure relief valve is in a plugged position in which the valve needle extends into the opening and seats against the valve seating to completely close the opening, closing communication between the load chamber and the low pressure drain. If fluid pressure within the load chamber exceeds the predetermined relief pressure, the pressure relief valve is opened and the valve needle lifts off the valve seating to relieve pressure in the load chamber.